Pipeline pipe and preparation method thereof

Abstract

The invention belongs to the field of pipes, and particularly discloses a pipeline pipe and a preparation method thereof. On the basis of specific components of the pipeline pipe, continuous casting, rolling and sizing procedures are matched, the microalloying state in the pipeline pipe is regulated and controlled, under the condition that the off-line normalizing procedure of the steel pipe after hot rolling is omitted, grains can be remarkably refined, the toughness is enhanced, the yield ratio is reduced, and the purposes of improving the toughness and impact allowance of the pipeline pipe are achieved; and the pipeline can still keep relatively low carbon equivalent, is high in weldability, and is particularly suitable for serving in low-temperature and extremely low-temperature environments.

Description

Technical Field

[0001] The invention belongs to the field of pipes, and in particular relates to a pipeline pipe and a preparation method thereof. Background Art

[0002] Pipeline pipes are widely used in many industrial fields such as petroleum, natural gas, chemical industry, water supply, heating, etc. In the oil and gas industry, pipeline pipes are mainly used to transport crude oil, natural gas and other media; in the chemical industry, they are used to transport various chemical raw materials and products; in the water supply and heating field, they are used to transport water and steam and other media. The particularity of pipeline service means that steel pipes often need to serve in extremely low temperature environments, requiring excellent low-temperature impact toughness. Most pipeline pipes are connected by welding, and the material is generally made of low-carbon equivalent micro-alloyed steel to ensure weldability. The commonly used pipeline steel grade is X52N. Although this type of steel pipe meets the minimum requirements of API 5L 46th standard, this type of steel pipe has low low-temperature impact toughness and small impact margin, which cannot meet the mechanical requirements of serving in extremely low temperature environments. In addition, conventional production pipeline pipes are generally smelted, cast pipe billets, perforated, rolled, offline normalizing or online normalizing, and then reduced to obtain pipeline pipes. Conventional processes involve offline normalizing or online normalization, which results in long production cycles and high energy consumption, and are unable to effectively reduce costs and improve efficiency. Summary of the invention

[0003] In view of the problems in the prior art that the pipeline has poor low temperature toughness and requires offline normalizing or online normalization during production, the present invention provides a pipeline and a preparation method thereof.

[0004] To achieve the above purpose, the following technical solutions are specifically included:

[0005] On the one hand, the present invention provides a pipeline pipe, comprising the following chemical components in percentage by mass: C 0.08-0.12%, Si 0.2-0.4%, Mn 1.3-1.5%, P≤0.016%, S≤0.005%, Cr≤0.08%, Mo≤0.04%, Ni≤0.03%, Cu≤0.045%, V 0.03-0.08%, N 0.01-0.025%, Nb0.015-0.035%, Ti 0.005-0.03%, Al 0.015-0.035%, and the balance is iron; the carbon equivalent of the pipeline pipe is ≤0.25%.

[0006] In the present invention, the content "%" indicating the chemical element composition of the line pipe means "mass percentage".

[0007] C is a carbide-forming element. As an important element for improving the strength of steel, the strength of steel increases with the increase of carbon content, but the impact toughness and elongation decrease significantly, and the toughness of the welding heat affected zone decreases. Therefore, the C content is controlled in the range of 0.08% to 0.16%, which can maintain the strength and toughness of the steel without adversely affecting the welding performance. The weldability of the pipeline is high.

[0008] Mn is an austenite forming element, which can improve the strength of steel pipes, improve the hardenability of steel pipes and reduce the quenching temperature. However, when the Mn content is too high, it will not only produce harmful MnS, but also cause grain coarsening and increase the sensitivity of temper brittleness, which is not conducive to the toughness of seamless steel pipes such as pipelines. Therefore, the upper limit of the Mn content is set to 1.5%, but in order to ensure the strength of the material, it is preferred to set the lower limit of the Mn content to 1.3%.

[0009] Si can remove oxygen from steel, thereby improving the toughness of the steel pipe. Too high Si content reduces plasticity, and too high Si content will affect welding performance. In addition, Si has poor heat permeability. Adding more Si will result in poor plasticity, and rolling defects such as inward folding and cracking are prone to occur during the billet rolling process. Therefore, as required for deoxidation, the minimum Si content is set at 0.2%, and the upper limit of the Si content is set at 0.4%.

[0010] V easily forms extremely stable carbides and nitrides with C and N in steel, and vanadium carbide and vanadium nitride usually exist in steel in a fine, dispersed form, so they can significantly refine the structure and grains of steel, increase the grain coarsening temperature, and reduce the overheating sensitivity of steel, thereby improving the strength and toughness of steel at the same time. However, V is expensive, so from an economical point of view, it is sufficient to achieve the corresponding performance and add as little V as possible.

[0011] The addition of N is harmful to most types of steel, but in V steel, it forms VN with V, which has a high solid solubility in austenite and a low solid solubility in ferrite. During the transformation of steel from austenite to ferrite, VN precipitates secondary in the steel to prevent grain growth and improve strength and toughness at the same time. If there is too little N element, the added V element will be dissolved in the ferrite matrix and will not play a strengthening role. Therefore, the addition of N element can save precious V metal. However, if the amount of N becomes excessive, the toughness in the heat affected zone of welding will deteriorate due to the presence of solid-solution N, affecting the welding performance. Therefore, the present invention controls the V content to 0.03-0.08% and the N content to 0.01-0.025%, so that V and N can achieve good VN micro-alloying and achieve the purpose of improving strength and toughness at the same time.

[0012] Nb can refine the grains. It forms carbides and nitrides with C and N in steel, inhibiting the growth of austenite in steel during heating. However, it is easy to be saturated. Therefore, the Nb content is designed to be 0.015-0.035%. If the Nb content is too high, the excess part exists in a free state. When the billet is rolled, cracks and external folds are likely to appear on the surface of the steel pipe.

[0013] Ti is a fine grain element, forming Ti(C, N) in steel, pinning austenite grain boundaries and preventing grain growth at high temperatures. It is dispersed in steel and can prevent austenite grain growth and refine grains. Ti has a low solid solubility in steel, and adding a trace amount can play a role in refining grains. In addition, TiN precipitates at high temperatures during continuous casting, and is difficult to be integrated into the matrix during subsequent rolling and heating. TiN particles can promote V(C, N) to precipitate on TiN particles at higher temperatures and at a faster rate in a composite precipitation manner, playing a nucleation role.

[0014] In addition, Nb, V, and Ti can combine with C (carbon) and N (nitrogen) in steel to form carbides, nitrides, and carbonitrides. These compounds dissolve at high temperatures and precipitate at low temperatures, thereby inhibiting grain growth and precipitation strengthening. However, excessive addition of Nb, V, and Ti will affect the toughness of the material. Therefore, the total mass percentage of V, Nb, and Ti is controlled to be ≤0.15%.

[0015] Al is a good deoxidizer in steel. It can cooperate with Si to remove oxygen from the raw materials and generate highly fine and ultra-micro oxides, which can effectively prevent the growth of grains when the steel is heated and refine the grains. Due to the saturation of Al's deoxidation effect and the avoidance of inclusions in the form of clusters, adding excessive Al to Cr-containing steel will form a denser layer of iron oxide on the surface of the steel pipe, which is not easy to remove under high-pressure water. Therefore, the upper limit of Al content is 0.03%. In addition, excessive addition of Al will form AlN with N in the steel, consume most of the nitrogen, and thus hinder the formation of VN in austenite.

[0016] When the C content is ≤0.12%, CE pcm Measuring and assessing carbon equivalence, CE pcm =C+Si / 30+(Mn+Cu+Cr) / 20+(Ni+Mo) / 60+V / 100, among which, CE pcm The content of each element is calculated by the above formula, where each element in the formula represents the corresponding element mass percentage; when CE pcm ≤0.25%, the pipeline can obtain high strength and toughness at the same time, and has good welding performance. pcm When the carbon equivalent CE is ≥0.24%, the welding performance of the pipeline will deteriorate. pcm≤0.25%, preferably carbon equivalent CE pcm ≤0.24%.

[0017] Preferably, the line pipe comprises the following chemical compositions in mass percentage: C 0.09-0.11%, Si 0.31-0.35%, Mn 1.39-1.45%, P≤0.016%, S≤0.005%, Cr 0.05-0.07%, Mo 0.01-0.03%, Ni 0.01-0.02%, Cu 0.03-0.04%, V 0.053-0.062%, N 0.015-0.022%, Nb 0.019-0.03%, Ti 0.01-0.02%, Al 0.02-0.03%, and the balance is iron.

[0018] Preferably, the line pipe has a yield strength of 360-420 MPa, a tensile strength of 510-560 MPa, an elongation of 47-49%, a yield strength ratio of 0.6-0.8, an impact value of 240-300 J at 0°C, a shear ratio of 90-95% at 0°C, and an impact value of 200-280 J at -20°C.

[0019] Preferably, the grain size of the line pipe is ≥ 8.5, that is, the grains in the line pipe reach the extremely fine grain grade required by the industry.

[0020] The line pipe of the present invention meets the X52N requirements in GB 9711-2017, has good toughness, large impact margin, high weldability, and is particularly suitable for service in low temperature and ultra-low temperature environments.

[0021] Preferably, the wall thickness of the line pipe is ≤12 mm.

[0022] The present invention also provides a method for preparing the pipeline pipe, comprising the following steps:

[0023] (1) The material is prepared according to the chemical composition of the line pipe, and then smelting, refining, vacuum degassing and continuous casting are performed in sequence to obtain a pipe blank;

[0024] (2) The tube blank is sequentially pierced, rolled, sized, air-cooled, and straightened to obtain a line pipe.

[0025] The preparation method of the line pipe of the present invention eliminates the online normalizing or deep cooling process, and utilizes the transportation time between the rolling and sizing of the steel pipe to promote the precipitation of V (C, N) in austenite, so as to obtain the nucleation core of the ferrite grains during the subsequent cooling. The TiN particles as the nucleation core have been formed in the continuous casting process. The TiN particles can promote the precipitation of V (C, N) on the TiN particles in a composite precipitation manner at a higher temperature and at a faster speed, and the precipitated V (C, N) particles further serve as the preferred nucleation position during the transformation of austenite to ferrite, thereby promoting the nucleation of ferrite. The V (C, N) precipitates are preferentially precipitated at the austenite grain boundary, which is equivalent to a nucleating agent, and can effectively refine the grains. The composite precipitation of V (C, N) particles under strain or on the first precipitate particles such as TiN can accelerate the precipitation of V (C, N) particles in the crystal. Therefore, based on the specific composition of the line pipe, the present invention cooperates with the continuous casting, rolling and sizing processes to adjust the TiN and V (C, N) phases. Even without reheating (offline normalizing), the grains can be significantly refined, the toughness can be enhanced, and the toughness and impact margin of the line pipe can be improved.

[0026] Preferably, in step (1), the smelting temperature is 1600-1630°C.

[0027] Preferably, in step (1), the refining temperature is 1630-1650°C.

[0028] Preferably, in step (1), the continuous casting temperature is 1550-1580°C.

[0029] Preferably, in step (1), when preparing the ingredients, the raw materials used include at least one of molten iron, scrap steel and pig iron, and the mass percentage of the molten iron is ≥50%.

[0030] Preferably, in step (2), the time interval between rolling and sizing is 60s-20min. The transportation time of the steel pipe in the continuous rolling and sizing process is between 60s-20min, which can promote the precipitation of V (C, N) in austenite to obtain the nucleation core of ferrite grains during subsequent cooling, which is beneficial to the strengthening and toughening of the pipeline.

[0031] Preferably, in step (2), the perforation temperature is 1200-1300°C. During the heating to the perforation temperature, there is no particular restriction on the heating rate, and a conventional heating rate may be used, such as a heating rate of 5-30°C / min.

[0032] Preferably, in step (2), the rolling temperature is 1260-1280°C.

[0033] Preferably, in step (2), the sizing temperature is 830-860°C.

[0034] Compared with the prior art, the present invention has the following beneficial effects: based on the specific composition of the pipeline pipe, the present invention cooperates with the continuous casting, rolling and sizing processes to control the microalloying state inside the pipeline pipe. Under the condition of eliminating the offline normalizing process of the steel pipe after hot rolling, the grains can be significantly refined, the toughness can be enhanced, and the yield ratio can be reduced, so as to achieve the purpose of improving the toughness and impact margin of the pipeline pipe. In addition, the pipeline can still maintain a low carbon equivalent and high weldability, and is particularly suitable for service in low and ultra-low temperature environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is the metallographic image of the pipeline in Example 1. DETAILED DESCRIPTION

[0036] In order to better illustrate the purpose, technical scheme and advantages of the present invention, the present invention will be further described below through specific examples. The test methods used in the examples and / or comparative examples are conventional methods unless otherwise specified; the materials, reagents, etc. used can be obtained from commercial channels unless otherwise specified.

[0037] Example 1

[0038] (1) Smelting: The raw materials for the batching include molten iron, scrap steel or pig iron according to the chemical composition of the pipeline in Table 1, and the mass percentage of the molten iron is ≥50%; then, an electric arc furnace is used for smelting, and the temperature of the molten steel is about 1620°C, which is then heated to 1640°C for refining outside the furnace, vacuum degassed, and argon gas is blown for more than 10 minutes, and then continuously cast at 1565°C to obtain a continuously cast round tube billet;

[0039] (2) Hot rolling: In a ring furnace, the tube blank is heated to 1260-1300°C, then taken out of the furnace for perforation (temperature is about 1200°C) to obtain a rough tube; at this time, the temperature of the rough tube is about 1100°C-1200°C, and then rolled at 1270°C by a limited mandrel continuous tube mill (MPM) or a three-roll limited mandrel continuous tube mill (PQF) to obtain a rough tube, at which the temperature of the rough tube is about 1050°C; the rough tube is directly transferred to a sizing and reducing device for sizing, the sizing temperature is 850°C, the transfer time between rolling and sizing is about 10 minutes, and air cooling is performed after sizing; and then straightening, water pressure testing and flaw detection are performed to obtain a line pipe, the size of which is φ323.9×10 mm.

[0040] Embodiment 2-5

[0041] The difference between Example 2-5 and Example 1 is that the ingredients of the line pipe are different, as shown in Table 1.

[0042] Comparative Examples 1-6

[0043] The only difference between Comparative Examples 1-6 and Example 1 is that the ingredients of the line pipes are different, as shown in Table 1.

[0044] Table 1 (% by mass, the balance is Fe)

[0045]

[0046]

[0047] Note: Cr, Mo, Ni and Cu are all impurity elements remaining from smelting and are not added intentionally. Moreover, the above elements have been taken into consideration in the calculation of carbon equivalent.

[0048] The pipelines prepared in the above embodiments and comparative examples were subjected to performance tests, and the test methods were as follows:

[0049] (1) The tensile properties, 0℃ impact and shear properties, and -20℃ impact properties of the pipeline are tested according to ASTM A370-24 standard, where the full-size specimen size of 0℃ and -20℃ is 10*10mm;

[0050] (2) The metallographic structure of the line pipe of Example 1 was observed using a metallographic microscope.

[0051] Table 2

[0052]

[0053] It can be seen from Examples 1-5 that the line pipe of the present invention includes the following chemical components in mass percentage: C 0.09-0.11%, Si 0.31-0.35%, Mn 1.39-1.45%, P 0.012-0.016%, S 0.002-0.005%, Cr 0.05-0.07%, Mo 0.01-0.03%, Ni 0.01-0.02%, Cu 0.03-0.04%, V 0.053-0.062%, N 0.015-0.022%, Nb 0.019-0.03%, Ti 0.01-0.02%, Al 0.02-0.03%, and the balance is iron. The pipeline pipe of the present invention has a yield strength of 360-420MPa, a tensile strength of 510-560MPa, an elongation of 47-49%, a yield strength ratio of 0.6-0.8, an impact value of 240-300J at 0°C, a shear ratio of 90-95% at 0°C, and an impact value of 200-280J at -20°C, which meets the requirements of X52N pipeline pipes. In addition, the method of the present invention produces X52N pipeline pipes without the need for an offline normalizing process after hot rolling, which can save the conventional offline normalizing process of steel pipes after hot rolling, save costs, and achieve low-cost production; and in the method of the present invention, the pipeline pipe contains specific contents of micro-alloying components such as V, N, and Ti, has a low carbon equivalent, good weldability, and a low yield strength ratio, and its impact toughness is significantly improved, the impact values ​​at 0°C and -20°C are high, and the shear ratio at 0°C is high, which is particularly suitable for service in severe cold and low temperature areas.

[0054] By the attached Figure 1 It can be seen that the grain size of the pipeline steel finally obtained by the present invention is small, the grain size is ≥8.5, and it reaches the ultra-fine grain grade.

[0055] It can be seen from Example 4 and Comparative Example 1 that as the carbon content increases, the strength is significantly improved, but the carbon equivalent also increases, the impact toughness at 0°C and -20°C decreases sharply, and the impact shear ratio is unqualified.

[0056] It can be seen from Example 2, Example 3 and Comparative Example 2 that no V element for grain refinement is added to the line pipe, and only a trace amount of V is introduced by smelting residues, and its strength fails to reach the lowest lower limit of strength in the X52N specification; it can be seen that the addition of V can effectively improve strength and impact through secondary dispersion precipitation.

[0057] It can be seen from Example 2, Example 3 and Comparative Example 3 that the strength of the line pipe is significantly improved by increasing the Mn content. However, the impact values ​​at 0°C and -20°C drop sharply, the impact shear ratio at 0°C is unqualified, and the yield strength ratio is high.

[0058] It can be seen from Example 4 and Comparative Example 4 that, within a certain range, increasing the Nb content in the line pipe reduces its strength and does not meet the strength requirements of the X52N specification. The impact performance will not be significantly deteriorated. However, at the rolling deformation temperature, the hard and brittle phase of Nb carbonitride is difficult to integrate into the matrix, and inward or outward folding is likely to occur during the rolling process, affecting the yield rate of the product.

[0059] It can be seen from Example 3 and Comparative Example 5 that when no N is added or a small amount of N is added to the line pipe, the grain size is not significantly refined, the strength of the line pipe is insufficient, and the impact toughness is reduced.

[0060] It can be seen from Example 2 and Comparative Example 6 that the microalloying element Ti is not added to the line pipe, the grains are slightly coarse, the strength and impact toughness of the line pipe are reduced, and the shear ratio is unqualified.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A line pipe, characterized in that: The pipeline comprises the following chemical components in percentage by mass: C 0.08-0.12%, Si 0.2-0.4%, Mn 1.3-1.5%, P≤0.016%, S≤0.005%, Cr≤0.08%, Mo≤0.04%, Ni≤0.03%, Cu≤0.045%, V 0.03-0.08%, N 0.01-0.025%, Nb 0.015-0.035%, Ti 0.005-0.03%, Al 0.015-0.035%, and the balance is iron; the carbon equivalent of the pipeline is ≤0.25%.

2. The line pipe according to claim 1, wherein: The invention comprises the following chemical components in percentage by mass: C 0.09-0.11%, Si 0.31-0.35%, Mn 1.39-1.45%, P≤0.016%, S≤0.005%, Cr 0.05-0.07%, Mo 0.01-0.03%, Ni 0.01-0.02%, Cu 0.03-0.04%, V 0.053-0.062%, N 0.015-0.022%, Nb 0.019-0.03%, Ti 0.01-0.02%, Al 0.02-0.03%, and the balance is iron.

3. The line pipe according to claim 1, wherein: The line pipe has a yield strength of 360-420 MPa, a tensile strength of 510-560 MPa, an elongation of 47-49%, a yield strength ratio of 0.6-0.8, an impact value of 240-300 J at 0°C, a shear ratio of 90-95% at 0°C, and an impact value of 200-280 J at -20°C.

4. The line pipe according to claim 1, wherein: The wall thickness of the pipeline pipe is ≤12mm.

5. A method for preparing a line pipe according to any one of claims 1 to 4, characterized in that: The steps include: (1) The material is prepared according to the chemical composition of the line pipe, and then smelting, refining, vacuum degassing and continuous casting are performed in sequence to obtain a pipe blank; (2) The tube blank is sequentially pierced, rolled, sized, air-cooled, and straightened to obtain a line pipe.

6. The method for preparing a line pipe according to claim 5, characterized in that: In step (1), when preparing the ingredients, the raw materials used include at least one of molten iron, scrap steel and pig iron, and the mass percentage of the molten iron is ≥50%.

7. The method for preparing a line pipe according to claim 5, characterized in that: In step (2), the perforation temperature is 1200-1300°C.

8. The method for preparing a line pipe according to claim 5, characterized in that: In step (2), the rolling temperature is 1260-1280°C.

9. The method for preparing a line pipe according to claim 5, characterized in that: In step (2), the sizing temperature is 830-860°C.

10. The method for preparing a line pipe according to claim 5, characterized in that: In step (2), the time interval between rolling and sizing is 60s-20min.

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